Process and device for cooling and rectifying gases in heat exchanger with a colder gas, particularly with (their) decomposition products



June 25, 1963 w. K. H. ULBRICH 3,095,292

PROCESS AND DEVICE FOR COOLING AND RECTIF'YING GASES IN HEAT EXCHANGER WITH A COLDER GAS, PARTICULARLY WITH (THEIR) DECOMPOSITION PRODUCTS Filed May 29, 1959 United States Patent 3,095,292 PROCESS AND DEVICE FOR COOLING AND REC- TlFYlNG GASES IN HEAT EXCHANGER WITH A COLDER GAS, PARTICULARLY WITH (THEIR) DECOMPOSITION PRODUCTS Wolfgang Karl Hermann Ulbrich, Munich, Germany, assignor to Gesellschaft fur Lindes Eismaschinen Aktiengesellschaft, Munich, Germany Filed May 29, 1959, Ser. No. 816,880 Claims priority, application Germany June 6, 1958 Claims. (Cl. 621-12) If a gas mixture is to be decomposed at low temperature by rectification, it is cooled in heatexchangers to the low temperature. The gas mixtures in question contain gaseous admixtures, which must be considered as impurities and which would interfere in the rectification. These impurities must therefor be eliminated from the gas mixture before the rectification proper starts. This can be done in a heat exchanger, if the impurities have a freezing point that is higher than the temperature to which the gas mixture is cooled in the heat exchanger and, due to the physical behavior of the gases used here, the purification of the gas mixture will be the more complete the more it is cooled, that is the colder the heat exchanger is. The desired purification of the gas mixture thus goes hand in hand with the desired cooling, that is, both operations can be preferably combined in a single operation. The impurities are deposited in the heat exchanger and must be removed again from there, otherwise the exchanger would be clogged in a short time. To make sure that the heat exchanger maintains its temperature necessary for cooling the gas mixture, decomposition products of the rectification, for example, a component, are passed through it in the opposite direction. If a component has been separated, for example, in the rectification, which is not needed for high purity, this component, which supplies cold to the heat exchanger must be colder, because of the temperature gradient to be maintained in the mutual process, than the gas mixture at this point. These two processes are carried out mutually in two cycles. Since this component is free of impurities when it enters the cold end, it has a certain absorption power for impurities, so that the latter dissolve again and issue at the hot end with the component.

But the absorption power for these impurities is reduced, because of the lower temperature of the component, so that no complete purification of the heat exchanger can be achieved by this mutual process. The need for operating the heat exchanger at the same time with one component, :and to remove with this component the impurities from the heat exchanger, is thus paradoxical, since the gas mus-t be cooler in each zone of the heat exchanger than the heat exchanger, and the latter again is colder than the gas mixture to be cooled, while for the purification, the component, which also serves as a scavenging gas, should actually be warmer than the gas mixture at this point, so that the absorption power for impurities is increased and all impurities can be removed. The present process of cold operation and simultaneous scavenging in one cycle is thus not reliable, so that other means had to be used, as they are described in the speci fication. With colder gas for cold-operation, the heat ex changer can not be completely scavenged, and with hotter gas for scavenging, cold operation is not possible. Both functions are contradictory, due to the known properties of the gases.

. The present invention relates to rectification of gas mixtures, at relatively low temperatures, and is concerned with an improved process of cooling and purifying a crude gas, in periodically reversible heat exchangers, by means of a colder gas derived from said crude gas. In

3,095,292 Patented June 25, 1963 performing this improved process, novel utilization of apparatus is involved.

In the decomposition of gas by cooling and rectification the gas in question is usually a gas mixture, e.g., air (called crude gas) which contains, in addition to the principal constituents, e.g., oxygen and nitrogen (called components), certain other gases, e.g., carbon dioxide, and vapors, e.g., humidity of the air (called impurities) in small quantifies, which latter are deposited in the heat exchanger during the cooling. For a continuous operation the heat exchangers must be provided in pairs and they must be periodically reversible. This applies in analogous manner to regenerators, counter-current coolers and reversing exchangers. In one switching period the crude gas is cooled as a crude gas current and purified, in other words, the heat exchanger is heated and contaminated; in another switching period the heat exchanger is cooled (cold-blown) and scavenged, in other words, the gasfrequently a decomposition productis heated and contaminated as a cooling and scavenging gas current. By the switch mechanisms on the heat exchangers the crude gas current and the cooling and scavenging gas current, respectively, are conducted through one heat exchanger each and maintained during one switching period.

In order to be able to perform the periodic reversals during a long operating period without the heat exchangers being clogged by impurities, a number of measures are, or may be, provided, such as: various types of branchofis of a part of the crude gas current from the heatexchangers, at intermediate temperature, and purification of the side-streams by chemical agents or other means; reheating; purification in counterflow to itself in a heat exchanger and subsequent cooling in counterflow to the scavenging gas in another heat exchanger; unbalanced flow; etc. These measures have one thing in common, namely, that all gas currents in question remain unchanged at least over the entire duration of a switching period. The elements switching the individual gas currents thus become active only when the period is reversed. In consideration of the general physical behavior of gases and under the conditions set out in the foregoing, the more the gas is cooled the more it is also purified, and for this reason cooling and purification of the crude gas during one and the same period is logical. However, as the sublimation power of the cold gas current decreases at the rate the temperature falls, simultaneous cold blowing and scavenging during the other period is, on principle, devoid of sense or logic. For eflicient cold blowing the temperature of the entering cold g'as current must be lower than the required temperature of the exiting crude gas stream.

These findings form the basis of the general inventive concept of the present invention. According thereto, the second switching period is subdivided in time into a scavenging partial period and a cold-blowing partial period, with more favorable prerequisites and conditions being provided for each partial period. This results in a process for cooling and purifying a crude gas.

This process alternates between. two periods, namely a cooling and purifying period and 1a scavaging and coldblowing period. In the cooling and purifying period a crude gas mixture is passed through one of two periodically reversible passageways of a periodically reversible heat exchanger, from the warm end to the cold end thereof, whereby this crude gas mixture is cooled and its gaseous impurities are frozen and deposited in solid state in said one passageway of said heat exchanger, prior to separation of the purified gas mixture into its components;

through the other of said two periodically reversible passageways of said heat exchanger, from the cold to the warm end thereof, whereby the heat exchanger is cooled and the impurities are scavenged and discharged at the warm end. The improved method for scavenging said impurity-containing passageway during said cold-blowing and scavenging period utilizes a first part of said coldblowing and scavenging period in which part the cold gas is passed through a separate passageway in the said heat exchanger in the opposite direction to that of the crude 'gas mixture, whereby this gas is warmed before it is passed through said impurity-containing passageway to be scavenged, and utilizes a second part of said coldblowing and scavenging period in which the cold gas is passed through said impurity-containing passageway, thereby cooling said heat exchanger in a known manner.

In carrying out this process, the method steps performed may utilize separate passageways in a heat exchanger during different parts of the two periods involved, or separate heat exchangers may be utilized. In either case, air may be the crude gas mixture employed.

The warmed scavenging gas current for the time-limited scavenging process can be produced by using the entire cold gas current for the heat absorption, or only a part of the cold gas current for the heat absorption and mixing with the other part, before the scavenging process starts. The last-mentioned step is probably more advantageous because of the better possibility of regulating the scavenging gas current by a simple control of the partial currents. Since the warmed scavenging gas not only expels the impurities but also heats the heat-exchangers to be scavenged, since they are to be cooled between two hot periods, a strict limitation in time of the scavenging process is particularly important, in order that there be sufficient time available for cold-blowing in one and the same switching period. The first part of said cold-blowing and scavenging period, in which the gas warmed in said separate passageway is passed through the impurity-containing passageway, is accomplished stepwise. During the first step of the first part of the period the warmed scavenging gas is passedin the cold end of the impurity-containing passageway. During the second step of the first part of the period, the gas is entered at a warmer zone of the. impurity-containing passageway, and during succeeding steps the gas enters at increasingly warmer zones of the impurity-containing passageway, whereby the second part of the cold-blowing and scavenging period, when cold gas is passed through the passageway, begins at the cold end of the passageway, after the first step of the first part of the period is completed. It is also of advantage for the above-mentioned purposes that, after the warm but veryintensive scavenging, the heat transfer from/the cold gas current to the second heat exchanger faces is much better in the cold-blowing period than if it were retarded, as is the ease up to now, by the deposited impurities.

The subdivision of the scavenging and cold-blowing period has also another substantial advantage. Up to now, the impuritieswere distributed throughout in the total amount of gas of the derived cold gas current. According to the process of the invention, the total amount of. impurities is contained only in the scavenging gas current, which represents quantitatively only a fraction of the cold gas current, so that the latter remains free of impurities during the partial cold-blowing period. According to another object of the invention, part of the gas used for cold blowing can be obtained as a pure product by discharging the contaminated gas through one pipe and, the scavenging process being complete, by Withdrawing the pure gas through another pipe.

The apparatus for carrying out the process consists of heat exchangers provided with periodically reversible passageways and inlets and outlets for the crude-gas and cold-gas current, and is characterized by a supply and control device for the scavenging-gas current, into the passageways, which can be adjusted, if necessary, in time and/or in amount. In particular, the apparatus is characterized by passageways, for heating the scavenging gas current, which are in heat-exchange relation with the passageways for the crude-gas current. In order to produce the hot scavenging gas current from the cold gas current, a feed pipe is provided from the supply for the cold gas current to the passageways for heating the scavenging gas current in the passageways and a discharge pipe from these passageways into the supply for the cold gas current, at a point closer to the passageways or in the passageways themselves. A special embodiment of the device is characterized by a discharge pipe for the heated scavenging gas current, which discharge pipe is branched to points of the passageways having different temperature zones.

The subject of the invention will be described more fully on the basis of the disclosure in the appended drawings in which FIG. 1 is a diagrammatic representation of an apparatus arrangement operable for use in carrying out the improved rectification process of the present invention, in which arrangement a periodically reversible heatexchanger is sufficient for the cooling of the gas mixture to such an extent that it can be led to a rectifying column (not illustrated), and

FIGS. 2-6 inclusive are diagrammatic representations of apparatus arrangements for use in carrying out modified rectifying and cooling procedures in accordance with the present invention, in which arrangements the regenerators are always arranged in pairs so that the gas mixture, after cooling within a regenerator, can then also be conducted directly to the rectifying column.

The figures illustrate only that part of the decomposition installations for gas mixtures, e.g., for air at low temperature-installations which are known per se-- which relates to the present invention, namely, the heat exchanger for cooling the impure gas mixture, with the impurities being condensed in solid form, and for the return of a decomposition product which serves as cooling agent for this gas mixture and at the same time removes the condensed impurities from the heat-exchanger. The path of the other decomposition product which is to be obtained at a very high degree of purity-for example, the oxygen in case of decomposition of air-is not depicted with reference to the heat-exchanger, for reasons of clarity: it is processed in a manner known per se.

FIG. 1 shows a heat exchanger of the counterfiow or reversing exchanger type at a certain moment of a switching period. A crude gas mixture is conducted through a supply pipe 1, into a passageway 2, and from there through a discharge pipe 3, while the cold gas, consisting, for example, of a decomposition product, is conducted through a supply pipe 4, into a passageway 5, and into a discharge pipe 6. In the supply pipe 4 there is arranged a flow path element 7, by which the cold gas can be conducted through a junction pipe 8 1nto a separate passageway 9, and through a junction pipe 10, again into a supply pipe 11 between the flow path element 7 and the cold end of the heat exchanger itself. In order to scavenge the impurities deposited and separated from the crude gas mixture during the preceding switching period in the passageway 5, the cold gas can be warmed by the indicated position of the fiow path element 7 in the separate passageway 9 and conducted through the passageway 5, with increased sublimation power. After the scavenging is completed, the flow path element 7 is switched again for the direct passage of the cold gas from the supply pipe 4 into the supply pipe 11.

FIG. 2 shows a heat exchanger of the regenerator type, in which a heating coil 12 is part of a separate passageway for heating the scavenging gas. Otherwise, the arrangement and method of operation corresponds to that of FIG. 1.

FIG. 3 shows a heat exchanger of the regenerator type, wherein a heating coil 13 is designed for a partial current produced by throttling in a pilot valve 14, warmed to a high temperature, and then mixed again with the other partial current for scavenging. After the scavenging the pilot valve 14 is opened wide for cold blowing, so that the partial current through the heating coil 13 is stopped.

FIG. 4'shows a regenerator of particularly advantageous simplicity and regulating capacity. In the supply pipe 4 for the cold gas current there is arranged a branch 15 to junction pipe 8 to a heating coil 16. At the warm end, a circulation blower 17 is provided in the junction pipe 10, and the junction pipe 10 is connected again, at 18, with the supply pipe 4 for the cold gas current. When the circulation blower is turned on, a scavenging gas current is produced which is warmed in the heating coil 16 and which is added at 18 to the cold gas. The scavenging gas current depends, with regard to temperature and amount, on the easy-to-influence capacity of the circulation blower 17. The arrangement of switchable inserts in cold zones can be completely foregone.

FIG. shows a heat exchanger which operates in a similar manner with regard to the production and control of the scavenging gas current as that of FIG. 4. However, the warmed scavenging gas can be supplied to the passageway to be scavenged at different points, that is, in different cross sections 31 and 32 of its length. In this case the partial periods of scavenging and cold blowing for difierent cross sections of the heat exchanger are preferably effected at different times.

Behind the circulation blower 17, there are arranged path pilot valves 19, 20 and from these different discharge pipes, 10, 21, 22 are connected to various cross sections of the passageway of the heat exchanger. This results in a number of variants in the switching and regulation of the warm scavenging gas current, so that the one with the best action under the prevailing conditions has to be found, for example, scavenging died with minimum warming of the heat exchanger.

FIG. 6 shows a regenerator with a warming device 24 for the warm scavenging gas current with three circulation blowers 25, 26, 27 and three outlets 28, 29, 30 to three different cross sections of the heat exchanger. Due to the elastic regulability, the requirements for an intensive explusion of the impurities andsufiicient cooling of the regenerators can be met effectively and at low cost.

The switch mechanism by which the passageways 2 and 5, FIG 1, can be reversed periodically has not been'represented in the drawings. Where and how, for example, in what temperature zones, the separate passageways 9, 12, 13, 16, 23, 24 are arranged in the heat exchanger depends on the respective conditions. Additional passageways, which are provided in known manner in various ways in heat exchangers, do not impair the applicability of the subject of the invention.

I claim:

1. In a process alternating in two periods, namely a cooling and purifying period and a scavenging and coldblowing period, in which cooling and purifying period a crude gas mixture is passed through one of two periodically reversible passageways, of a heat-exchanger in the direction from the warm end to the cold end thereof, whereby said crude gas mixture is cooled and its gaseous impurities are frozen and deposited in solid state in said one passageway of said heat-exchanger, before the purified gas mixture is separated into its components by rectification at low temperature, and in which cold-blowing and scavenging period a cold gas, preferably a component of said gas mixture, is passed through the other of said two periodically reversible passageways of said heat-exchanger, said other passageway containing frozen gaseous impurities deposited during a preceding cooling and purifying period; in the direction from the cold end to the warm end thereof, whereby the heat-exchanger is cooled and said deposited impurities are scavenged and discharged at the warm end, the improved method for scavenging said other passageway during said cold-blowing and scavenging period comprising a first part of said cold-blowing and scavenging period in which the cold gas is passed through a separate passageway in said heat-exchanger in the opposite direction to that of the crude gas mixture, whereby said gas is warmed, before it is passed through said impurity-containing passageway to be scavenged, and a second part of said cold-blowing and scavenging period in which the cold gas is passed through said other passageway, thereby cooling said heat-exchanger in a known manner.

2. In a process alternating in the a cooling and purifying period and a cold-blowing and scavenging period, in which cooling and purifying period a crude gas mixture (e.g. air) is passed through one of a pair of periodically reversible heat-exchangers of the regenerator type in the direction from the warm end to the cold end thereof, whereby said crude gas mixture is cooled and its gaseous impurities are frozen and deposited in a solid state in said one of said two regenerators, before the purified gas mixture is separated into its components by rectificationat low temperature and in which cold-blowing and scavenging period a cold gas, preferably a component of said gas mixture, is passed through the other of said two regenerators in the direction from the cold end to the warm end thereof, whereby said other regenerator is cooled and said deposited impurities are scavenged and discharged at the warm end of said other regenerator, the improved method of scavenging said impurity-containing regenerator during said oold-blowing 'and scavenging period comprising a first part of said period in which the cold gas is passed through a separate pasageway said regenerator in the opposite direction to that of the crude gas mixture, whereby said gas is warmed before it is passed through said other regenerator to be scavenged and a second part of said period in which the cold gas is passed through said other regenerator directly, thereby cooling it in a known manner.

3. The improved process defined in claim 1, further characterized in that the entire amount of said cold gas is passed through, and thereby warmed up in, said separate passageway of said heat-exchanger.

4. The improved process defined in claim 2, further characterized in that the entire amount of said cold gas is passed through, and thereby warmed up in, said separate passageway of said regenerator.

. 5.- The improved process defined in claim 1, further characterized in that only -a part of said cold gas is passed through said separate passageway of said heat-exchanger, whereby it is war-med up, and then is mixed with the remaining other part of said cold gas before passing the total of said cold gas through said impurity-containing passageway to be scavenged.

6. The improved process defined in claim 2, further characterized in that only a part of said cold gas is passed through said separate passageway of said regenerator, whereby it is warmed up and then mixed with the remain ing other part of said cold gas before passing the total of said cold gas through said regenerator to be scavenged.

7. The improved process defined in claim 1, further characterized in that said first part of said cold-blowing and scavenging period, in which said gas warmed in said separate passageway is passed through said impurity-containing passageway, is accomplished stepwise, so that during the first step of said first part of said period said warmed scavenging gas is passed in at the coldest zone, i.e. the cold end, of said impurity-containing passageway, during the second step of said first part said gas is assed in at a warmer zone of said impurity-containing passageway, .and subsequently during next steps said gas is passed in at increasingly warmer zones of said impurity-containing passageway whereby said second part of said coldhlowing and scavenging period, in which said cold gas is passed through said passageway, begins .at the cold end of two periods, namely,

7 said passageway when said first step of said first part of said period is completed.

8. The improved process defined in claim 2, furthercharacterized in that said first part of said cold-blowing and scavenging period, in which said gas warmed in said separate passageway is passed through said regenerator to be scavenged, is accomplished stepwise, so that during the first step of said first pant of said period said warmed gas is passed in at the coldest zone, i.e. the cold end, of said regenerator, during the second step of said first part said gas is passed in at a warmer zone of said regenerator, and subsequently during next steps said gas is passed in at increasingly warmer zones of said regenerator whereby said second part of said period in which said cold gas is passed through said regenerator begins at the cold end of said regenerator when said first step of said first part of said period is completed.

9. An apparatus for the separation of a gas mixture by rectification at low temperature which comprises: a heatexchanger provided with periodically reversible passageways, one of them periodically passed by a crude gas mixture to be cooled and purified from its gaseous impurities whereby said impurities are frozen and deposited in a solid state, another one periodically passed by a cold gas, preferably by a component of said gas mixture, thus coldblowing and scavenging said passageway containing impurities deposited during a preceding cooling and purifying period, and a separate passageway arranged parallelto said periodically reversible passageway; supply and discharge pipes for both the gas mixture and the cold gas; periodically operating control devices in said supply and discharge pipes; a branch-off in said supply pipe for the cold gas to the cold end of said impurity-containing passageway; a first junction-pipe connecting said branch-off and the cold end of said separate passageway; a second junction pipe connecting the warm end of said separate passageway and said supply pipe for the cold gas to the cold end of said impurity-containing passageway, the connection of said second junctionfpipe to said supply pipe being positioned closer to said cold end of said impuritycontaining passageway than said branch-off of said first junction-pipe; and a separate control device for passing cold gas through said, separate passageway, said second junction-pipe is further branched-off whereby the further branched-,ofi parts of said second junction-pipe are connected to points in difierent zones of said impurity-containing passageway to be scavenged and said separate control device, divides the gas flow from the warm end of said separate passagewaytosaid:impurity-containing passageway in the remaining part of said second junctionpipe or in one or another of said branched-off parts of said second junction-pipe.

10. An apparatus for separation of a gas mixture by rectification at low temperature, which comprises: a pair of periodically reversible heat-exchangers of the regenerator type, one of them periodically passed by crude gas mixture to be cooled and purified from its gaseous impurities whereby said impurities are deposited in a solid state and the other periodically passed by cold gas, preferably by a component of said gas mixture, thus cold-blowing and scavenging said regenerator; a separate passageway in each of said regenerators; supply and discharge pipes for both the gas mixture and the cold gas; periodically operating control devices in said supply and discharge pipes; a branch-off in said supply pipe for the cold gas; a first junction-pipe connecting said branch-oil? and the cold end of said separate passageway; a second junction-pipe connecting the warm end of said separate passageway and said supply pipe for the cold gas, the connection of said second junction-pipe to said supply pipe being positioned closer to the cold end of said regenerator than said branch-off of said first junction-pipe; a separate control device for passing cold gas through said separate passageway, and said second junction-pipe from the warm end of said separate passageway to said supply pipe of the cold gas having branched 01f parts therefrom, whereby the branched: off parts of said second junction-pipe are connected to cross-sections in different zones of said regenerator to be scavenged and said separate control device divides the gas flow from the warm end of said separate passageway to said regenerator to be scavenged in the remaining part of said second junction-pipe or in one or another of said parts branched off from said second junction-pipe.

References Cited in the file of this patent UNITED STATES PATENTS 2,534,478 Roberts Dec. 19, 1950 2,653,455 Etienne Sept. 29, 1953 2,663,167 Collins Dec. 22, 1953 2,699,047 Karwat et al. Jan. 11, 1955 2,863,295 Newton Dec. 9, 1958 FOREIGN PATENTS 1,214,369 France Nov. 9, 1959 

1. IN A PROCESS ALTERNATING IN TWO PERIODS, NAMELY A COOLING AND PURIFYING PERIOD AND A SCAVENGING AND COLDBLOWING PERIOD, IN WHICH COOLING AND PURIFYING PERIOD A CRUDE GAS MIXTURE IS PASSED THROUGH ONE OF TWO PERIODICALLY REVERSIBLE PASSAGEWAYS, OF A HEAT-EXCHANGER IN THE DIRECTION FROM THE WARM END TO THE COLD END THEREOF, WHEREBY SAID CRUDE GAS MIXTURE IS COOLED AND ITS GASEOUS IMPURITIES ARE FORZEN AND DEPOSITED IN SOLID STATE IN SAID ONE PASSAGEWAY OF SAID HEAT-EXCHANGER, BEFORE THE PURIFIED GAS MIXTURE IS SEPARATED INTO ITS COMPONENTS BY RECTIFICATION AT LOW TEMPERATURE, AND IN WHICH COLD-BLOWING AND SCAVENGING PERIOD A COLD GAS, PREFERABLY A COMPONENT OF SAID GAS MIXTURE, IS PASSED THROUGH THE OTHER OF SAID TWO PERIODICALLY REVERSIBLE PASSAGEWAYS OF SAID HEAT-EXCHANGER SAID OTHER PASSAGEWAY CONTAINING FROZEN GASEOUS IMPURITIES DEPOSITED DURING A PRECEDING COOLING AND PURIFYING PERIOD; IN THE DIRECTION FROM THE COLD END TO THE WARM END THEREOF, WHEREBY THE HEAT-EXCHANGER IS COOLED AND SAID DEPOSITED IMPURITIES ARE SCAVENGED AND DISCHARGED AT THE WARM END, THE IMPROVED METHOD FOR SCAVENGING SAID OTHER PASSAGEWAY DURING SAID COLD-BLOWING AND SCAVENGING PERIOD COMPRISING A FIRST PART OF SAID COLD-BLOWING AND SCAVENGING PERIOD IN WHICH THE COLD GAS IS PASSED THROUGH A SEPARATE PASSAGEWAY IN SAID HEAT-EXCHANGER IN THE OPPOSITE DIRECTION TO THAT OF THE CUDE GAS MIXTURE, WHEREBY SAID GAS IS WARMED, BEFORE IT IS PASSED THROUGH SAID IMPURITY-CONTAINING PASSAGEWAY TO BE SCAVENGED, AND A SECOND PART OF SAID COLD-BLOWING AND SCAVENGING PERIOD IN WHICH THE COLD GAS IS PASSED THROUGH SAID OTHER PASSAGEWAY, THEREBY COOLING SAID HEAT-EXCHANGER IN A KNOWN MANNER. 